Yes.

Once one postulates science advanced enough for FTL ships, anything is possible.  With techs including shields, energy weapons, fusion, anti-matter, black hole whatevers, etc., there would seem to be little in the way of limits.

Residence buildings a mile-cube each, orbiting factories, deeply-buried structures ....

For example, a single mile-high residence block would have 528 stories, each level would be 27,878,400 sq feet.  Assume 28 floors for utilities and services, assume 2,878,400 sq feet for shafts, give each adult 1000 sq feet, and one gets 25,000 adults per floor for 12,500,000 adults per building.  Assume most adults live with a child, and that would make it 20 million per square mile building.  (Thus, a family of 4 would have 2,000 sq feet.)

Assume one such building in each 20 square miles of land, and one gets one million per square mile.

Dedicate the continent of Australia to human habitation and reserve the rest of the planet for nature, etc.  Australia is almost exactly 3 million square miles.  That would allow 3 million million people on Earth, or 3 trillion.

Alternatively, convert West Virginia to similar use and one could house over 24 billion with the rest of the planet not habitated by humans.

It only takes sufficient science and technology.

I'm not sure any civ would build someplace so potentially unstable unless there was some great need of it.  However, one could postulate, I suppose, that the intelligent species desired the pressure or the great depth for some specific reason, perhaps cultural if not physiological.

Here's some math.  First, though, let's postulate that the species wants no structure to be get closer than three miles to the surface of the water, say, 15,000 feet depth.  I make this premise because the average depth of the Pacific Ocean is 13,760 feet, so there would be no need to use a trench unless the "normal" Pacific Ocean floor depth were not deep enough.  The Pacific is the deepest ocean, on average.

The Mariana Trench data on the web is that it is 1,580 miles long, 43 miles wide, and 36,201 feet deep (at its deepest point).  Making some simplifying assumptions, I'll calculate on the basis of 1000 miles long, 40 miles wide, and a useable depth of 3 miles (30,000 feet average depth but no structure can be within 15,000 feet of surface).

1000 x 40 x 3 = 120,000 cubic miles for habitation.

Extending my math from the previous post, I will postulate 1 million population per cubic mile (it had been square mile previously).  That results in 120 billion in population.

Note that 22 ocean Trenches have been discovered to date, of which 9 are considered "major" (google "oceanic trench").

So, if your species had more than 120 billion, needed more volume per individual, wanted more geo separation of groups (nationalities?), or needed a greater minimum depth, there would be significantly additional room for them elsewhere.  For example, the Peru-Chile Trench has a max depth of 26,460 feet and extends 3700 miles.  The Kuril Trench has a max depth of 34,587 feet and extends 1,800 miles north-south.

Google Russia (or any place) and multiply by 1 million per square mile.

Maybe all their parents liked raising babies?

Tech consistent with FTL tech would seem to ease most of those restrictions.  For example, the Earth's core is molten but slowly cooling, so just move the waste heat there and stabilize it!  Or build orbit height radiator fins.

There are already far too many people on this planet for all citizens of the Earth to enjoy Western standard of living. The solution : A Restricted Family Policy (0 to 2 children) in 64 countries.

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Camron Sareja

It is not possible to sustain multiple billions of human-sized life-forms on an Earth-sized planet while maintaining ecological balance.  Clean and efficient energy production helps marginally.  The problem is food, not energy.  It takes massive amounts of territory to grow enough food for that many people, and utilizing entire regions for the purpose (as with the American Midwest) is irrevocably ecologically devastating.  "I don't get the impression they fret much over their carbon footprint or where the next glass of water is coming from." - This is a naive mindset and exactly the same kind that leads to the grotesquely incorrect notion that technology can circumvent the natural order.  Technology is a tool, not a "free lunch" pass.  Expanding from one ecological system into another does nothing to resolve the underlying issues of sustainability and scarcity.  That is exactly the same principle as the "spoils of war" economic model in which a lack of warfare bringing valuables in from outside results in a total economic collapse.  Economies, like ecologies, are closed circuits, and to think of them otherwise is just a case of rampant denial.

The question is not whether a sapient species can support itself on such a world at such numbers for some time, it's a question of how long, and the answer is a few dozen centuries from beginning to end, give or take.  Eventually, wildlife will have nowhere else to go, and die out.  The other life-forms in the system that depended on them will die out.  As more food is produced, more people are born to use up the food.  It's a vicious cycle of starvation and breeding.  The only way out of the cycle is to accept that there's only so much carrying capacity no matter HOW good our technology is.

Only ONE potential technology will ever change that, period, and that's assuming the technology is even possible.  If it becomes possible to arrange atoms at-will into whatever appropriate molecular configuration we require, we'll no longer need agriculture, and can "live off the land" in perhaps the most literal way imaginable.  All sources of the elements of organic materials will yield whatever we need them to yield, so we can siphon tiny bits from everywhere without destabilizing or overtaxing the ecosystem.  But even then, it would at best quadruple the carrying capacity of a world, meaning a total of 8 billion or so.  Until that time though, planetary carrying capacity is severely limited if there is any interest in not damaging ecosystems and maintaining the existence of civilization.

The thing is, the ecosystem is already correctly balanced, and that balance has a maximum capacity and duration.  Upset the balance, lower the duration ... increase the capacity, upset the balance.  It's all inextricably linked.

You seem to be assuming that all technologies are possible.  It's the same kind of thinking that dominates most concepts of aliens; anything is possible.  On worlds where the global average climate is similar to any particular average region on Earth, life on that world will be recognizable in comparison to life here.  There won't be polar bears at the poles, but there might well be creatures that strongly resemble them.  There will be no 6-legged cows or flying beavers or whatnot.  And technologically developed intelligent alien life will be almost exclusively humanoid, and thus frequently mammalian if not primate (or likely rodents, too), primarily because our brains developed as a result of developing fine dexterity.  A species that can't easily and finely manufacture and manipulate tools will not increase intellectually because of lack of opportunity.  There might be exceptions, but they'll be rare at best.  Even life based on other elements like silicon will look strange, but still be generally comparable in form and function on a macro-ecological level.  Only radically different environments in which life still springs up (which would seem to be rare if our planet-discovery trends continue as-is) will produce truly unrecognizably alien creatures.

If, and I stress "IF", it becomes technologically possible (energy cost-effective) to directly convert energetic particles into conventional matter, then the issue would be completely resolved, period.  The question then becomes strictly a matter of humanity filling up areas better suited to other terran lifeforms.  With that technology, we're better off abandoning the planet and living in space entirely if we want to have large populations, since planetary colonies would be irrelevant.

If, and again I stress "IF", it becomes technologically possible (and energy cost-effective) to rearrange preexisting atoms into molecular structures of our choosing at will, we'll still be dependent on semi-conventional sources (frequently planets) for the raw materials needed, and that will again limit our maximum capacity, but otherwise still negate our overall impact as long as we're under that limit.  This is because consuming more of those elemental materials still removes resources from the ecosystem and a 15 lb replicated turkey still requires 15 lbs (plus whatever resource used to power the process) of material to make.

If we discover fusion power, we'll still require vast amounts of fuel for them at our present consumption rates.  If we somehow crack antimatter, we'll need significantly less fuel, probably a quarter at most, but that's still a lot at our present consumption rates.  If some other completely undreamt of method that requires NO fuel of any kind is discovered, then THAT would radically change things too, but again to the point where we may as well just let the other life-forms have planets since they'd be irrelevant to us.

It may be possible that we discover fuel-less energy production, and/or two-way matter/energy conversion, but those technologies would completely eliminate the question of carrying capacity because they would neutralize all tethers and obligations to planetary existence.  Lesser versions of similar technologies would increase a world's carrying capacity, but still not nearly to the levels seen in the game, if for no other reason than people have to live somewhere on or under the surface, and unless we live miles below the crust, something else will already be living there that we'll have to displace.  And the more species we displace and the greater the degree to which we do so, the more we'll adversely affect the ecosystem.  Consider also the massive global infrastructure that would be necessary to support technologies of that type on that scale.  it's not even remotely reasonable to conclude that 20 billion people could live on Earth without consequence even with such technologies.

How a population arrives at a world is not relevant to this discussion though.  The point is that any single world cannot support more than 10 billion people without a severely detrimental impact on the ecosystem.  Firstly because of resource requirements, namely food; and secondly because of the requirements of living space.

The issue of food can be balanced out by absolute molecular control, or completely neutralized with matter-energy conversion if either can be developed and become cost-effective.  But the problem of living space has a hard cap.  Every home, every business, all facilities, to say nothing of the support infrastructure needed to make it all work, will take up a minimum amount of space per person no matter how much nanotechnology turns wristwatches into supercomputer/weapon/sensors.  You and your clothes, knick-knacks, furniture, and so on will always take up a minimal amount of space.  Now, through various design advances that can be reduced drastically, there's still a minimum threshold and It's not a particularly convenient way to live for a society that has managed to lick the interchangeability of matter and energy.

Now...this could be resolved with technologies that create pocket space, or bends space to create the functional appearance of more space, but either that will have practical limits, or will again neutralize our dependence on planetary living.  This is what it ultimately boils down to; either we'll live on planets and limit the population that can live on each planet, or we'll develop technologies that neutralize both problems and simply not live on planets because there'd be no point.

Finally though, this is assuming FTL travel is possible at all.  If (and I stress "IF") it IS possible in some way, the energies required to make use of it will be immense, far beyond even the scope of matter/antimatter reactions, it would require the annihilation of a Jupiter-sized mass (so technically two such masses, since one must be antimatter) just to send a ship of a size we could currently build.  In which case it would be a long time before such travel became commonplace enough to transplant large numbers of people, and even then such journeys between stars would be monumental undertakings.  If (and I continue to stress "IF") FTL travel is possible, it will never become the "set a course and push the 'go' button" kind.  That said, there's really nothing wrong with relativistic travel.  A variety of potential advances could one day put most of the speed of light within our reach, and in that the problem is only for those left behind, not those leaving.  I figure using relativistic travel, it'd take roughly 150,000 years to settle the entire galaxy, though of course to the colonists it would be a fraction of the time.

I'm going to have to defer to LTJim on this one-I did run the numbers quite some years ago, and while I seem to remember the numbers being a tad lower (my theoretical limit for the landmasses of our planet was around 350-450 trillion), it's still relatively in the same ballpark.  I can only assume I was hypothesizing shorter residences.

As far as the argument of minimum living space goes, there's a couple videos of some people in Hong Kong, if memory serves, who manage to fit more into ~250 square feet than most of the rest of the world does in 2500.  It's a little claustrophobic, granted, but all the needs are met through rather ingenious methods, and it certainly shows the potential for improvement.  In any case, I think LTJim's estimate of 2,000 square feet per family of four is quite adequate enough for the job.

The other discussions are potentially more interesting-how to do things, and the FTL versus relativistic aspect-and if I weren't currently condemned (there's a nice word for it) to using only the library's computers I'd love to get involved.  As it stands, I'll have to let everyone else debate the relevant points, as I simply don't currently have the time.

My apologies if I've offended anyone, and additionally because I no longer have access to my work on theoretical population numbers, and therefore I'm not quite sure how I arrived at the number I did.

More opinions stated as facts:

"... will inevitably lead to violence..."

"...will invariably lead to ..."

"It's blatantly irresponsible to ...."

Buildings can be deep and buildings can be tall.  The limits are technological.  You seem locked in 20th Century city planning.  I remember reading about how predictions around the beginning of the 20th Century (~1900) predicted that cities could not sustain populations above one million because the streets would fill with horse manure, and they had the 19th Century math to prove it!

Additionally, the post header was "while sustaining ecological diversity" not whatever metric you're trying to cite.  It is difficult for me to see how humans confining themselves in a single continent like Australia fails to maintain planetary ecological diversity.

Maybe the folk in some future want to live on a planet because they like natural gravity, much like so many today crave organic food.  They may prefer all-natural force vectors on their bodies, rather than artificial ones with who knows what differences from nature.

As to your opinions voiced at the end of your post, it seems clear to me is that you have not read Larry Niven's stories with the race called The Puppeteers.

The question has a background. Thing is that my husband was telling me that a Muslin men can marry many women if he wishes so & it is allowed by their religion. I told him that time the ratio between men & women wasn’t balanced. The number of women was high & Islam say’s that it is a must for a man/woman to get married or lese it is considered as sin. For that reason, one man could marry more than once, as if no one left unmarried.

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Camron Sareja

You keep saying I'm stating opinion as fact as if somehow it ONLY invalidates my points.  I'll admit in the speculation department I'm playing it closer to the chest than you seem to be.  Much as I like technology, I find it naive to think that all things are possible and that technology inherently solves problems (to which our current world is glaring proof to the contrary).  But we're ALL speculating here, I'm just being a little more realistic about it.  Not every advance in science is going to be another breakthrough.  We're getting into a creeping doom (not that we're necessarily doomed, yet) phase of advancement.  There will be no quantum computers, no FTL travel, and no artificial gravity (aside from spinning modules or perhaps some strange system of magnets to polarize your body like an MRI and "pull/push" you toward the "floor").

As to the second bit, I'm just really not sure how to respond by this point.  You've basically put yourself firmly in the camp of people that I'm constantly trying to defend against excessively religious types.  The religious ones keep saying science is just another form of faith, and I keep telling them why it's not, and then someone comes along and spouts a bunch of Star Trek-style nonsense about what's possible in the future, and then all my defense of science is undone.

My point is that you CAN'T use Australia as an example.  You want to usurp an entire continent from its indigenous lifeforms?  Or even a quarter of it for that matter is too much.  That IS an opinion I suppose, but hey, that justification has always worked before, so why not now.

This second to last bit of yours comes right back to that you think technology has far more potential than it really does.  But suffice to say, I already pointed out that I accept that some radically different technologies will exist in the future (FAR in the future) and that I don't really know what they'll be exactly, since no one can.  But to say something as asinine as, "They'll figure out something, I mean c'mon, they'll be future-people after all."  This is exactly the same thing as religious fundamentalists who oppose environmental reform because God would never allow us to ruin the planet, and also as those who claim it'll be the duty of later generations to start changing how we do things.

Your comment about my FIFTH paragraph is the only reason I replied again at all, though this will be the last time anyway since we aren't getting anywhere, so if you want the last word go for it then.  Your response seems a relatively clear indicator that your answer would've been "no", but of course I can't be sure of that.  I chose to be optimistic about it though.  If you had said yes, I wouldn't have replied (no doubt now you might be wishing you had said yes then), because I haven't got any interest in arguing with crackpots.  No sane, rational person would ever believe that a race capable of traveling from one star to another would be so obvious about it and then STILL not make contact.  One expects that kind of "did not/did so" behavior from children and criminals, not starfaring species who have managed to become a type 2 civilization.  I can't see much reason for them to bother coming here in the first place, but xenobiological studies and xenoanthropological studies might be sufficient motivation, or just general stellar surveying.  Oh and before I forget, don't mention the prime directive, because that would be silly.  It always made sense to me to hide from races that haven't yet left the ground, let alone gone to space, but the advent of interstellar travel as a prerequisite is simply silly.  Once a race has begun to travel to the other planets of their system, they're as ready as they'll ever be for first contact as a culture.  The only thing that will affect our preparedness to meed another species will be if we somehow find evidence of them BEFORE any have made first contact.  I know, you said start another thread, but since this is the only discussion there will be on the matter, I figured it wouldn't kill anyone.

 Hello all!

I like to ask a random question: would it be possible to house over one million Human-sized beings in an area of less than one hundred square miles?